The present invention relates to a system and a method respectively for reducing interference in cellular communication systems, particularly systems using Code Division Multiple Access (CDMA). The invention also relates to a cellular communication system implementing such a method.
In a cellular communication system each cell is served by a base station controlling the mobile stations within the respective cell. In a CDMA-system all users use the same frequency, i.e. all users can use the whole bandwidth and time simultaneously. (In for example TDMA--(Time Division Multiple Access) systems, the users in contrast thereto are separated through different time slots and frequencies). In a CDMA-system, however, all users use the same frequency and time, and they are only separated by different user specific codes. It is nearly impossible to make the users orthogonal to each other which has as a consequence that the users will interfere. It is therefore important that signals from all users arrive at the base station with approximately the same power. The interference from other users will only be reduced by the spreading factor, which is the number of chips a transmitted data bit is divided into.
In a CDMA-system it is of extreme importance that the transmission power is controlled in an efficient manner. The transmission power of a mobile station is suggested to keep the symbol-to-interference ratio (SIR) at a constant level. If, however, the interference level in the system increases, the transmission power will also be increased, which sometimes is referred to as the cocktail party effect. If a mobile station, which is controlled by a base station, transmits with a transmitting power which interferes another base station, the latter base station runs the risk of being blocked if the level of interference is high. Such blocking is unacceptable for but very short periods of time.
The transmitting power of a mobile station is regulated through measuring the power, received at the base station, and transmitting power control commands back to the mobile station, for example via a control link. This enables the base station to receive all mobiles which are connected to it with approximately the same signal strength. However, since a CDMA-system has a frequency reuse factor of 1, the same frequency is used also in the adjacent base station. Under normal circumstances this is not a problem, since the mobile stations will be connected to the closest base station or rather the base station giving the best quality for uplink as well as for downlink connections. However, sometimes mobile stations are not connected to the "closest" radio base station.
In U.S. Pat. No. 5,430,889 an automatic power control system for a mobile radio system is disclosed in which a control center is provided. Via said control center a plurality of base stations are linked together. Mobile stations are arranged to transmit an identification code unique to each mobile station and a data sequence common to all mobile stations but transmitted by each mobile station at a nominally different time within each burst. Each base station comprises correlator means for discrimination between signals from mobile stations within its own cell and interference signals from mobile stations in adjacent cells. A data logger is provided for storing the results of the correlations so that details of relative power levels in times of reception are recorded. The control center furthermore includes a signal processor responsive to the data logger at each base station for identifying mobile stations involved in potential mutual interference situations and feeding back appropriate instructions to the base stations concerned. A suitable power control signal is then transmitted to such mobile stations. This invention is based on continuous sending of information in order to prevent interference situations from occurring. However, these continuous transmissions in themselves contribute in producing, if not interference, at least noise. Furthermore power is consumed due to the transmission of identification codes and data sequences at all times. Thus, a system as disclosed in U.S. Pat. No. 5,430,889 is not satisfactory because of the above mentioned deficiencies. The system furthermore is quite complex.